1. Field of the Invention
This invention relates to a vaccine for porcine reproductive and respiratory syndrome (PRRS) comprising one or more strains of attenuated porcine reproductive and respiratory syndrome virus (PRRSV).
A previously unknown disease of pigs, PRRS was recognized for the first time in the United States in North Carolina in 1987. From there, it quickly spread throughout all of the major swine producing areas of North America. It next appeared in Europe, and today PRRS has almost worldwide distribution. Many swine producers, government officials, and veterinarians believe that PRRS is currently one of the most serious economic threats faced by the swine industries worldwide.
As its name implies, PRRS is characterized clinically by its ability to cause reproductive failure (in pregnant females, especially when initially infected late in gestation), and respiratory tract illness (in pigs of all ages, but most common and severe in young pigs). Infection with PRRSV is also thought to be immunosuppressive and thus potentiate the effects of other swine pathogens. On the basis of retrospective serological studies, it also has become evident that many infections of swine with PRRSV are either subclinical or result in less obvious clinical signs. Therefore, the PRRSV often gains access to a herd and spreads extensively before its presence is first detected.
The virus can persist in an infected host for at least several months. Such "carriers" perpetuate the infection and make control of the disease extremely difficult. As a consequence, the most effective means for reducing the economic impact of PRRS is to vaccinate (immunize) potentially susceptible pigs before they are exposed to virulent field virus.
2. Description of the Prior Art
Only a single attenuated vaccine, (manufactured by Boehingher Ingelheim and distributed by NOBL Laboratories) prepared from a single strain of PRRSV, is commercially available. It is only licensed for use in pigs between 3 and 18 weeks of age for the prevention of respiratory tract illness (Gorcyca et al., 1995).
Another attenuated vaccine has been described for the prevention of reproductive failure (Hesse et al., 1996 ). It is prepared from a single strain of PRRSV and has only been tested against a single strain of PRRSV. The challenge strain is described as heterologous on the basis of the anamnestic response of vaccinated gilts following challenge; however, no other evidence has been presented to establish that the two strains, i.e., the one used for the vaccine and the one used for challenge of immunity, are genetically or antigenically different.
There are two known major serotypes of PRRSV (Done et al., 1996, herein incorporated by reference). One (prototype Lelystad) is representative of at least most strains that have been isolated in Western Europe. The other (prototype ATCC 2332) is representative of at least most strains isolated in North America and Asia. The two prototypes are easily distinguished serologically in that antisera from pigs exposed to one prototype react poorly or not at all with the other. There also are antigenic variants within prototypes (Meng et al., 1995), and base sequence differences among strains isolated in North America have allowed for their differentiation (Wesley et al., 1996).
A study, wherein sows previously exposed to PRRSV strain NADC-8 were protected when subsequently exposed during gestation to either virulent strain Lelystad or virulent strain NADC-8 (Lager et al., 1995), provided evidence that at least one American isolate could protect against the reproductive failure caused by either an American or European isolate of PRRSV.